Abstract

We present a novel approach for achieving tightly concentrated optical field by a hybrid photonic-plasmonic device in an integrated platform, which is a triangle-shaped metal taper mounted on top of a dielectric waveguide. This device, which we call a plasmomic light concentrator (PLC), can achieve vertical coupling of light energy from the dielectric waveguide to the plasmonic region and light focusing into the apex of the metal taper(at the scale ∼ 10nm) at the same time. For a demonstration of the PLCs presented in this paper, we numerically investigate the performance of a gold taper on a Si3N4 waveguide at working wavelengths around 800nm. We show that three major effects (mode beat, nanofocusing, and weak resonance) interplay to generate this light concentration phenomenon and govern the performance of the device. Combining these effects, the PLC can be designed to be super compact while maintaining high efficiency over a wide band. In particular, we demonstrate that under optimized size parameters and wavelength a field concentration factor (FCF), which is the ratio of the norm of the electric field at the apex over the average norm of the electric field in the inputting waveguide, of about 13 can be achieved with the length of the device less than 1μm for a moderate tip radius 20nm. Moreover, we show that a FCF of 5 – 10 is achievable over a wavelength range of 700 – 1100nm with the length of the device further reduced (to about 400nm).

Figures (9)

(a) Schematic of an compact PLC that is a hybrid photonic-plasmonic structure with a gold triangle taper integrated on top of a Si3N4 ridge waveguide with a SiO2 buffer layer. (b) The top view of this hybrid structure.

(a) The cross section and top view of a hybrid photonic-plasmonic waveguide. (b) Two supermodes (HTM,0 and HTM,1) come from the superposition of the fundamental TM-like mode (TM0) of the purely photonic waveguide and the fundamental symmetric mode (S0) of the purely plasmonic waveguide. (c) Two supermodes (HTE,0 and HTE,1) come from the superposition of the fundamental TE-like mode (TE0) of the purely photonic waveguide and the fundamental asymmetric mode (A0) of the purely plasmonic waveguide. The electric field lines are sketched for these modes.

(a) The normalized electric field profiles of TM0, S0, HTM,0 and HTM,1. (b) The normalized electric field profiles of TE0, A0, HTE,0 and HTE,1. The width w of the Au layer is 620nm. The wavelength λ is 800nm. The effective index neff for each mode is listed.

Dispersion characteristic of the four supermodes (HTM,0, HTM,1, HTE,0 and HTE,1) of the hybrid photonic-plasmonic waveguide in Fig. 2(a) in the form of the real and imaginary parts of the mode effective index versus the width w of the Au strip. All other dimensions are the same as those in Fig. 2(a). The wavelength λ is 800nm.

The normalized electric field profiles of HTM,0 and HTM,1. The width w of the Au strip is 60nm. While the HTM,0 field in highly concentrated in the metallic region, the HTM,1 field has considerable strength outside the metallic region and becomes more similar to the TM0 mode of the corresponding purely photonic waveguide.

Normalized electric field patterns in the planes horizontally (Y = 320nm) and vertically (X = 0) cutting through the Au layer. The length L of the Au triangle is 900nm, and the width W is 400nm. The calculated field concentration factor (FCF) is 12.6 with the radius of curvature a at the tip being 20nm. Q is the apex point of the triangular taper.

FCF versus length L of the Au triangle: the maximum width W is 300nm for the blue solid curve and 400nm for the red dashed curve. The wavelength λ is 800nm. The radius of curvature a at the tip is 20nm. All other parameters are the same as for the tapered structure shown in Fig. 6.

Spectra of transmission T, reflection R and the sum of the two for the Au taper length (a) L = 1μm, and (b) L = 2μm. The maximum width W of the taper is 400nm. The radius of curvature a at the tip is 20nm.

Spectra of transmission and FCF for three groups of lengths of the Au triangle (a) 0.4μm, 0.425μm and 0.45μm, (b) 1μm, 1.025μm and 1.05μm, and (c) 2μm, 2.025μm and 2.05μm. The width W of the triangle is 400nm. The radius of curvature a at the tip is 20nm. All other parameters are the same as for the tapered structure shown in Fig. 6.